A shadow mask type color cathode-ray tube conventionally includes a cathodoluminescent display screen comprising a mosaic pattern of phosphor deposits on the inner surface of the CRT's faceplate. The deposits typically are arranged in sets of three, called triads, and have either a round or elongate shape. For convenience, the deposits will be referred to herein as "dots". A plurality of electron guns controlled by an associated scanning system directs a corresponding number of electron beams toward the display screen through a multiapertured shadow mask positioned between the electron guns and the display screen. Each beam's path through the mask is such that it impinges upon and excites only one type of color phosphor material.
The shadow mask consists of a membrane of electron beam absorbing material containing a very closely patterned array of holes of the same shape as the phosphor deposits. These holes are required to be accurately positioned relative to the corresponding groups of phosphor dots on the display screen so as to permit passage only of the particular beam that is intended to address a designated type of phosphor material. Portions of the beam that would fall outside the dot area are intercepted and absorbed by the mask. Accordingly, it is necessary for the shadow mask to be constructed very precisely and positioned very accurately within the CRT.
The mosaic phosphor dot pattern normally is formed by a direct photoprinting process that proceeds substantially as follows: A screen area on the inner surface of the faceplate is first coated with a photosensitive slurry containing phosphor particles of one color. Next, the tube's shadow mask is mounted on the faceplate temporarily and the coating is exposed to ultraviolet light projected through the mask holes from a source located at a position corresponding to that of the related electron gun. The shadow mask is then removed and the coating is treated to remove the unexposed portions, leaving a pattern of dots of the one phosphor color. These steps are then repeated for each remaining color to deposit a group of three phosphor dots--a red, a green and a blue--on the faceplate opposite each mask hole. Of critical importance to this process is the ability to precisely relocate the shadow mask in the same position relative to the faceplate each and every time it is removed. Any variation in the relative placement of the shadow mask and faceplate will cause misplacement of the phosphor dots and color errors in displayed images.
Conventionally, the shadow mask is removably mounted within the CRT by a plurality of spring clip fasteners attached to a frame surrounding and supporting the mask. Before the fasteners are initially secured in place, the shadow mask is properly located relative to the face plate, as by placing a spacer between them. While maintaining this relationship, the clips are engaged with receiving slots in the face plate and then welded to the mask frame. Each clip is thereby precisely located on the shadow mask frame so that it mates with a specific slot the that particular faceplate. The spring clips are yieldable to allow the mask assembly to be removed from the faceplate during tube processing and later replaced in the same position.
Although the use of spring clip fasteners for shadow mask mounting is quite satisfactory and has long been the standard mounting method in entertainment grade color CRTs, it has been found to be unsatisfactory in certain special applications, as where significant levels of vibration are experienced or color accuracy requirements are very high. One example is in CRTs intended for use in avionics applications. The accuracy and readability of in-flight information displayed by such a CRT is essential for safe operation, and any display distortion caused by aircraft vibration must be minimized or eliminated. Because spring clips are inherently susceptible to vibration, and thus may permit relative movement between the shadow mask and display screen, their use is preferably avoided in CRTs designed for use in such applications.